An adequate use of finite resources is one of the greatest challenges of our times. To address this, lightweight concepts based on continuously fiber reinforced composites (FRC) are already being adapted for the transportation industry, especially within the automotive and the aerospace sectors. In order to broaden the use of lightweight composite structures and components, suitable processing, monitoring and control techniques are required for a variety of materials, constituting a prerequisite for economic, flexible and automated high volume production. In this regard, photonic technologies can provide valuable solutions. In this presentation, the latest developments within the field of FRC laser machining are summarized. For the processing of large structures such as resin transfer molding parts, combinations of galvo scanners with robots or axis systems are of particular interest. For this purpose, both high brightness cw fiber lasers and pulsed systems are used. Within the repair chain for valuable FRC parts, pulsed UV and NIR lasers are used for the precise removal of fiber layers in order to generate a defined scarfing. For both applications, disintegration of the fiber matrix interconnection due to thermal impact has to be avoided. Thermoplastic composites are becoming increasingly important for many industrial applications. In contrast to thermoset systems, welding techniques are particularly applicable. In this context, laser welding is not limited to the joining of transparent-absorbing-combinations, as it is required for conventional laser transmission welding processes but can be extended to the welding of structural parts consisting of high-performance carbon fiber reinforcements.
Today, fiber reinforced materials are present in a wide field of industrial applications. Short glass fiber reinforced composites are mainly used in automotive, aerospace and medical sectors. In recent years, endless fiber reinforced thermoplastics have gained importance as construction material, especially for lightweight assembly. There are different methods of joining thermoplastic materials such as vibration, resistance and induction welding. Another process is laser transmission welding, which can be characterized by its excellent reproducibility, high flexibility and potential for automation. Typically, laser transmission welding can be applied for joining unreinforced or glass fiber reinforced thermoplastic parts. This welding process was now adapted to heat conduction welding for joining thermoplastic CFRP to itself. The goal of these investigations was to determine the influence of the matrix material on the weld seam quality. The experiments were conducted with a carbon fiber fabric reinforced polyphenylene sulfide with natural matrix material as well as containing carbon black. In the first step, the temperature distribution at the upper joining member, where the heat generation occurred, was evaluated. The heat affected width was determined and correlated to the process temperatures in order to develop a process model. In a next step, lap shear samples were welded and tested. These results were then correlated with previous results.
In the aviation industry, a major market for carbon fibre reinforced plastics (CFRP), <40.000 drilling operations are performed throughout the assembly process of a small aircraft. Additionally, the drive to minimize costs and time are prevalent in the manufacturing process. The quality requirements in the aviation industry are set to a high level and drilling tools have to be changed frequently, causing considerable costs in terms of tooling and time losses. Laser processing offers benefits such as flexible, and wear free cutting, which contributes to the optimization of processing costs. In this investigation a laser machine, process control, processing strategies and handling equipment adapted to high precision macro drilling and low cycle times were presented. The setup included a novel short pulsed high power laser source by TRUMPF Laser GmbH emitting at λ = 1030 nm integrated in a 5-axis machine. The lab-state laser source provides pulses at tp = 20 ns, at a maximum pulse energy of Ep = 100 mJ and a maximum average power of Pavg = 1.5 kW, while maintaining a very good beam quality, allowing small focus diameters. Due to a large variety of parameters that have an influence on the process, a test plan based on design of experiments was applied to identify ideal parameter fields. Parameters optimized towards high ablation rates and orthogonal kerf angles were identified. The results revealed a promising industrial processing option for high quality macro boreholes.
Considering the increasing amount of data for communication and infotainment applications, fabrication of optical networks and bus systems is a challenging task for production engineering. A two-step manufacturing process for polymer optical waveguides is presented. By improving the highly efficient flexographic printing technology by laser functionalization of the printing tool in combination with a subsequent spray application, high-quality waveguides are accomplished. By adjusting the resulting surface energy of the foil substrate in the first fabrication process, the spray application achieved high-aspect ratio waveguides with a low attenuation of 0.2 dB/cm at 850 nm.
Optical data communication is increasingly interesting for many applications in industrial processes. Therefore mass production is required to meet the requested price and lot sizes. Polymer optical waveguides show great promises to comply with price requirements while providing sufficient optical quality for short range data transmission. A high efficient fabrication technology using polymer materials could be able to create the essential backbone for 3D-optical data transmission in the future. The approach for high efficient fabrication technology of micro optics described in this paper is based on a self-assembly effect of fluids on preconditioned 3D-thermoformed polymer foils. Adjusting the surface energy on certain areas on the flexible substrate by flexographic printing mechanism is presented in this paper. With this technique conditioning lines made of silicone containing UV-varnish are printed on top of the foils and create gaps with the exposed substrate material in between. Subsequent fabrication processes are selected whether the preconditioned foil is coated with acrylate containing waveguide material prior or after the thermoforming process. Due to the different surface energy this material tends to dewet from the conditioning lines. It acts like regional barriers and sets the width of the arising waveguides. With this fabrication technology it is possible to produce multiple waveguides with a single coating process. The relevant printing process parameters that affect the quality of the generated waveguides are discussed and results of the produced waveguides with width ranging from 10 to 300 μm are shown.
Pulsed laser sources with pulse durations in the millisecond regime can be used for spot welding and seam welding of aluminum. Seam welds are generally produced with several overlapping spot welds. Hot cracking has its origin in the solidification process of individual spot welds which determines the cracking morphology along the seam welding. This study used a monitoring unit to capture the crack geometry within individual spot welds during seam welding to investigate the conditions for initiation, propagation and healing (re-melting) of solidification cracking within overlapping pulsed laser welds. The results suggest that small crack radii and high crack angles with respect to welding direction are favorable conditions for crack healing which leads to crack-free seam welds. Optimized pulse shapes were used to produce butt welds of 0.5 mm thick 6082 aluminum alloys. Tensile tests were performed to investigate the mechanical strength in the as-welded condition.
The application of thin borosilicate glass as interposer material requires methods for separation and drilling of this material. Laser processing with short and ultra-short laser pulses have proven to enable high quality cuts by either direct ablation or internal glass modification and cleavage. A recently developed high power UV nanosecond laser source allows for pulse shaping of individual laser pulses. Thus, the pulse duration, pulse bursts and the repetition rate can be set individually at a maximum output power of up to 60 W. This opens a completely new process window, which could not be entered with conventional Q-switched pulsed laser sources. In this study, the novel pulsed UV laser system was used to study the laser ablation process on 400 μm thin borosilicate glass at different pulse durations ranging from 2 – 10 ns and a pulse burst with two 10 ns laser pulses with a separation of 10 ns. Single line scan experiments were performed to correlate the process parameters and the laser pulse shape with the ablation depth and cutting edge chipping. Increasing the pulse duration within the single pulse experiments from 2 ns to longer pulse durations led to a moderate increase in ablation depth and a significant increase in chipping. The highest material removal was achieved with the 2x10 ns pulse burst. Experimental data also suggest that chipping could be reduced, while maintaining a high ablation depth by selecting an adequate pulse overlap. We also demonstrate that real-time combination of different pulse patterns during drilling a thin borosilicate glass produced holes with low overall chipping at a high throughput rate.
Continuous carbon fibre reinforced plastics (CFRP) are recognized as having a significant lightweight construction potential for a wide variety of industrial applications. However, a today‘s barrier for a comprehensive dissemination of CFRP structures is the lack of economic, quick and reliable manufacture processes, e.g. the cutting and drilling steps. In this paper, the capability of using pulsed disk lasers in CFRP machining is discussed. In CFRP processing with NIR lasers, carbon fibers show excellent optical absorption and heat dissipation, contrary to the plastics matrix. Therefore heat dissipation away from the laser focus into the material is driven by heat conduction of the fibres. The matrix is heated indirectly by heat transfer from the fibres. To cut CFRP, it is required to reach the melting temperature for thermoplastic matrix materials or the disintegration temperature for thermoset systems as well as the sublimation temperature of the reinforcing fibers simultaneously. One solution for this problem is to use short pulse nanosecond lasers. We have investigated CFRP cutting and drilling with such a laser (max. 7 mJ @ 10 kHz, 30 ns). This laser offers the opportunity of wide range parameter tuning for systematic process optimization. By applying drilling and cutting operations based on galvanometer scanning techniques in multi-cycle mode, excellent surface and edge characteristics in terms of delamination-free and intact fiber-matrix interface were achieved. The results indicate that nanosecond disk laser machining could consequently be a suitable tool for the automotive and aircraft industry for cutting and drilling steps.
We report on ablation experiments of sputter deposited thin film systems of NiCr and Al2O3 for the fabrication of strain
sensors. To ensure proper functionality of the electrical circuits, the metal film has to be selectively removed while
damage in the Al2O3 films has to be avoided. Damage thresholds of the Al2O3 layer are investigated and damage
mechanisms are discussed. Damage thresholds decrease with increasing number of scans until reaching a constant value.
The processing window defined as the ratio of Al2O3 damage threshold and NiCr ablation threshold increases with
increasing film thickness and number of scans.
This paper presents results of ablation experiments of NiCr layers with thicknesses ranging from 23nm to 246nm on
Al2O3 substrates. Investigated parameters are fluence, number of pulses, film thickness and substrate roughness. The
influence of the parameters on the removal threshold is analyzed in order to identify stable processing parameters.
Patterned NiCr thin films as an essential component for the measurement of mechanical stress are required for the
development of sputtered thin film strain gages. With this new approach strain sensors will be resistant against creeping
or swelling through changing ambient conditions unlike conventional strain gages.
Laser double pulses offer interesting opportunities to increase the ablation performance of ultra short laser pulses. In
recent published and performed experiments we have presented an optical setup that covers delay times from some
picosecond up to 20 ns as well as first experimental results of ablating aluminium and silicon. In this paper we present
further results of especially interesting time domains for both materials. The ablation efficiency on silicon with inter
pulse delays from 6.3 ns to 15 ns was investigated. In this range the double pulse effect was mainly depending on the
fluency. The double pulse efficiency increase is connected with a higher thermal impact on the work piece. The change
of delay and repetition rate has no influence on the ablation efficiency for both single and double pulses. The
experiments on aluminium concentrated on the pulse delays of 50 ps to 400 ps. The ablation depth per pulse is lower
than for single pulse ablation in this range. Double pulse efficiency decreases up to a pulse delay of 150 ps.
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